FA PHONE SendVerificationCode

        //the method is sending verification code
        //the country id is concatenated
        //you can take the country id as user input as well
        private void sendVerificationCode(String mobile) {
            PhoneAuthProvider.getInstance().verifyPhoneNumber(
                    "+" + mobile,                 //phoneNo that is given by user
                    60,                             //Timeout Duration
                    TimeUnit.SECONDS,                   //Unit of Timeout
                    TaskExecutors.MAIN_THREAD,          //Work done on main Thread
                    mCallbacks);                       // OnVerificationStateChangedCallbacks
        }